Gary
Skinner
Stephen
Tomkins
In the pink:
Colour from carotenes
Think of amplification and you probably
imagine a band on stage next to huge speakers.
But things other than sound can be amplified,
and not just by electronics.
I
n nature, bioamplification causes substances to
become more concentrated as they move from
eater to eaten along a food chain. You may have
heard this story in relation to the concentration of
pesticides like DDT along a food chain, and how
this causes problems for those animals, like birds
of prey, at the top (see Box 1).
However, bioamplification can be more
benign than that. It leads to much of the colour
in nature. For example, in birds, there are three
main sources of their (often striking) colouration:
melanin pigmentation, structural colouration and
carotenoid pigmentation. Melanin is made in the
animal’s own body and structural colours come from
the way feathers interact with light, but carotene
pigments come, in many cases, from the diet and
are amplified along a food chain and concentrated
in parts such as feathers, beak or skin.
Key words
Box 1 Bioamplification
The concentration of DDT, a pesticide still in
use in some parts of the world, is amplified
up a food chain by a factor of 10 million or
more.
Bioamplification also gave rise to Minamata
disease in Japan. Mercury-containing
compounds from industrial processes were
released into the sea. They accumulated
up the food chain until people who ate
contaminated tuna became seriously ill. Of
the 10 000+ known victims, over 1800 died.
food chain
food web
photosynthesis
spectrum
DDT in fish-eating birds 250 ppm
DDT in large fish 25 ppm
DDT in small fish 2 ppm
DDT in zooplankton 0.5 ppm
DDT in phytoplankton 0.04 ppm
DDT in the water 0.000 003 ppm
ppm = parts per million
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Salt lake life
First then to the world’s salt lakes, the most famous
probably being the Great Salt Lake in Utah, USA.
At certain times of year this lake has a green
appearance in the water due to vast numbers of a
tiny alga called Dunaliella (Figure 1).
Brine shrimps, Artemia, one male and one female
(with eggs). Brine shrimps live in inland salt lakes but
not in the sea.
This carotene helps to protect the embryos inside
the cysts, which can stay viable for many years.
Figure 1 The green colour of this lake is caused by
the presence of the Dunaliella alga whose cells contain
chlorophyll..
Figure 2 The red colouring of these salt ponds in San
Francisco Bay is due to the presence of Dunaliella.
As plants, these algae make food in the process
of photosynthesis and are at the base of a short
food chain in this and other similar lakes. From
the glucose they make in photosynthesis, with the
addition of a few minerals, they can make everything
else they need. Included in what they make is a
pigment called carotene, found in abundance in
nature, famously in carrots. This carotene is most
obvious in the older algal cysts, which are reddish
and, in huge numbers, give the lake water a red
colour too (Figure 2).
Box 2 Carotene as a protective
antioxidant
Most molecules from which living things
are made are subject to oxidation, and this
can produce free radicals which, in turn,
can damage cells, leading to such diseases
as cancer. The cysts of brine shrimps, being
highly desiccated, are very susceptible to
oxidation and free radical damage, but this is
avoided by high levels of carotenoid pigments.
In one short food chain the brine shrimps are fed
on by flamingos (although not in the Great Salt
Lake). These birds have a highly specialised beak
to be able to take advantage of this abundant, but
tiny, food. The beak acts as a sieve allowing the bird
to process large volumes of water and extract a lot
of food in a short time (Figure 3). The carotene
ends up in the flamingos and, yes, is the source of
their pinky-orange colour.
Up the food chain
The main consumer of the algae is a small
crustacean called the brine shrimp (Artemia salina).
These are one of the few animal species which can
live in the very salty water, and they therefore get
most of the available algal food. Their numbers
are, as a consequence, vast. At a point in their
life cycle, the Artemia also form cysts containing
concentrated carotene which gives the egg-like
cysts an orange colour.
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Catalyst
Figure 3
Cristiano Ribeiro/Julia Saponova/ Ivan Mikhaylov/Bigstockphoto
make them in the first place? The simple, and not
surprising, answer is photosynthesis. The carotenes
are, of course, pigments and as such they absorb
light, the very thing plants need to do to make
sugar from carbon dioxide and water. Chlorophyll
is the main light absorbing pigment, but it only
absorbs red and blue light, hence its green colour
– see Figure 4.
Figure 4 Plants look
green because they
reflect the green part
of the visible light
spectrum.
Flamingos are born white and gain their pink colour
from the carotenes in their food which they take in
An absorption spectrum shows more clearly the
extent to which different wavelengths of light are
absorbed by a pigment. The absorption spectrum of
chlorophyll is shown in Figure 5a; the two ‘bumps’
show that chlorophyll absorbs strongly at the low
and high wavelength ends of the spectrum.
through their filter-feeding beak.
ABSORPTION
ABSORPTION
When flamingos are kept in
A#HLOROPHYLL!BSORPTION3PECTRUM
BB#AROTENE!BSORPTION3PECTRUM
captivity, as they often are, this
OF6ISIBLE,IGHT
OF6ISIBLE,IGHT
specialised diet is difficult to
provide, and they will feed on
other things. They would not
be pink if they were not given a
dye in their artificial food. Zoos
usually add something called
Roxanthin Red to the food, and
indeed there are companies
who supply Flamingo Food (e.g.
Flamingo Fare) which contains
7AVELENGTHNM
7AVELENGTHNM
this and other carotene-related
compounds.
But the role of carotenoids in making nature Figure 5 Absorption spectra of (a) chlorophyll, and (b)
beautiful does not end with flamingos. The red bill β-carotene.
of the zebra finch, the yellow one of the blackbird,
the yellow of canary feathers and many of the
All the green and yellow light is reflected and thus
colours we find in fruits and vegetables are, at least
wasted. It is not surprising to realise that carotenes
in part, due to carotene and related compounds.
absorb different coloured light from chlorophyll;
The role of carotene and its relatives in birds was
they are after all a different colour. The carotene
first proved in 1934. Canaries were fed on an
absorption spectrum is shown in Figure 5b. This
artificial diet free of carotenes and ended up with
shows that the carotene is filling in some of the
white feathers!
wavelengths that the chlorophyll does not absorb.
Plants have many such accessory pigments which
Plants and carotenes
absorb light energy and pass it to chlorophyll for
So plants make carotenes, which animals cannot photosynthesis.
do, and then animals acquire these and use
them to protect themselves against free radicals
(Box 2) and to produce mate attracting colours, Gary Skinner is biology editor of Catalyst. Stephen Tomkins
warning colours and so on. But why do the plants teaches biology at Cambridge University.
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